This invention relates to solid state power controllers and particularly to indication circuitry for indicating the status of the power controller while using all solid state components.
Solid state power controllers are generally known that exhibit the performance that is the equivalent of an electromechanical circuit breaker but using no mechanical elements. Such circuit breakers or power controllers are disclosed, for example, in copending applications Ser. Nos. 023,361 and 023,368, respectively, by Billings et al. and Mitchell et al. and both filed Mar. 23, 1979 and assigned to the present assignee, now respectively issued as U.S. Pat. Nos. 4,245,184, and 4,245,185, Jan. 13, 1981. Such apparatus is highly useful and effective and may be made more effective by combining it with additional means for providing an electrical signal indicative of the status of the power controller, namely whether the power controller is ON and conduction should be occurring between the supply and load with which it is associated or whether the power controller is OFF, such as by having been tripped out, so that no conduction is occurring in the load circuit. The generation of a positive electrical signal indicative of these different states is very useful. One way in which it can be used is to indicate the status to the operator, such as by lighting a lamp in the event of either an ON or an OFF signal. Alternatively, such a signal may be applied to logic circuitry or other control circuitry in order to initiate some more complex sequence of events such as attempts to initiate turn on in the event of an OFF signal upon determination of any number of criteria.
Status indication in solid state power controllers has in the past been accomplished by the use of miniature relays. In this manner, a normally closed (NC, or Form "B") relay circuit will permit active status indication of an OFF state because during the ON state, the normally closed relay will have a signal applied to it that results in its interruption and therefore the absence of the OFF signal. For ON state indication, it has similarly been suitable to use a normally open (NO, or Form "A") relay circuit so that the status indication occurs upon the application of the signal from the power controller that closes the normally open relay. Combinations of normally open and normally closed relays (Form "C") may indicate both ON and OFF states.
Miniature relays can perform active status indication for both ON and OFF states but are undesirable in some applications because they do not have the reliability, durability and environmental advantages of a solid state circuit.
In addition, it is known that some status indication can be achieved utilizing solid state devices that may be characterized as of the sinking type. These may be an output transistor, such as a conventional bipolar transistor, that for the ON state, or a logical "1", is energized by a base signal from the power controller so as to be conductive in its collector-emitter circuit. The OFF state, or logical "0" state, is indicated by the absence of the output transistor base signal so that its collector emitter circuit is non-conductive or virtually an open circuit.
The use of a single element such as a bipolar transistor to provide both ON and OFF state indications does not satisfy the need for active status indication of both states. That is because it passively indicates the OFF state by the absence of an output indication signal. Such an indication may result from a malfunction of one of the operative components as well as from the existence of the OFF state.
The present invention addresses the foregoing considerations and provides a unique solution in the form of a totally solid state indication circuit having the capability of providing an active OFF status signal. The disclosed circuit utilizes a depletion mode field effect transistor to provide the OFF indication. This known type of device has the characteristics of being normally conductive in the absence of a gate signal and is rendered non-conductive by the application of a gate signal. Thus, its use permits the working current in the status circuit between its source and drain to flow and provide an output during all times when the gate signal is not present as a result of the power controller being OFF.
In addition, arrangements are provided in which both ON and OFF active status indication is given. These include a combination of a depletion mode field effect transistor with another semiconductor device such as a bipolar transistor or an enhancement mode field effect transistor in a series arrangement in the status indication circuitry with a common branch therebetween. The circuit portion containing the depletion mode field effect transistor provides an active status signal upon OFF conditions of the power controller, that is, when its gate signal is absent. The other device, such as a bipolar transistor, will provide an active status signal of the ON condition during times when the control signal to its base is on.
In this way, new solid-state indication circuits analogous to relay circuits of either form B or form C can be provided. There is a positive conductive indication exhibited for each state.